The present invention relates to controls for an hydraulic system, and more particularly, to load sensing controls which permit the system to respond to a variety of types of input.
In recent years, the growing use of hydraulic systems has resulted in an increasing demand for more sophisticated and versatile controls for such systems. Quite naturally, such demand for better controls has resulted in attempts to apply electronic circuit technology as the logic input to control hydraulic systems.
One of the major difficulties in the use of electrical and electronic circuitry to control hydraulics is the selection of an appropriate interface between the electrical portion of the system and the hydraulic portion. One known type of interface is an electrically-actuated solenoid valve. However, if the hydraulic flow rates through the system are substantial, the flow forces acting on the solenoid valve make it necessary to use a fairly large, expensive solenoid having an excessive current draw. Therefore, the weight, expense and power requirements result in limited usefulness for such an interface.
Another known type of hydraulic-electrical interface is the nozzle flapper valve arrangement, which typically is used to generate a pair of pilot pressures, which bias the opposite ends of a main control spool. The precision required in producing a nozzle flapper valve having a reproducible, linear relationship between electrical input and hydraulic flow makes such an arrangement too expensive for a large segment of the hydraulic control market.
Accordingly, it is an object of the present invention to provide an hydraulic system which is adaptable to the use of electronic control logic at a cost which makes its potential use more widespread.
It is a related object of the present invention to provide an improved interface means to permit the use of electrical and electronic controls for hydraulic circuits.
As the use of hydraulic systems has grown, the recent interest in energy conservation has resulted in the development and adoption of load sensing hydraulics, i.e., hydraulic systems in which the load imposed on the system is sensed and the "load signal" is used to match the output of the fluid delivery source to the demand for fluid. The prior art has generally utilized the load sensing capabilities of hydraulic circuits for the fairly limited purpose described above, but have not used load signals, whether natural or synthetic, as a major element in the overall system control.
Accordingly, it is an object of the present invention to provide a load sensing hydraulic system in which the load signal is utilized as part of the main control, and as part of the electrical-hydraulic interface.
The above and other objects of the present invention are accomplished by the provision of an improved hydraulic system for controlling the flow of fluid from a variable fluid delivery source to a fluid actuated device. The system includes main control means disposed in series flow relationship between the fluid source and the fluid actuated device, the main control means including a main flow orifice. The flow through the main control means is normally a function of the area of the main flow orifice, with the pressure drop across the orifice normally being substantially constant. The variable fluid delivery source includes a load signal chamber and a means responsive to changes in the fluid pressure within the load signal chamber to vary the delivery of the fluid source. The system further includes means providing a load signal representative of the load on the fluid actuated device and a means communicating the load signal to the load signal chamber.
The improvement comprises a valve means disposed within the load signal communicating means. The valve means includes a first port in fluid communication with the load signal providing means, a second port in fluid communication with the load signal chamber, and a third port in fluid communication with a source of reference fluid, such as the system reservoir. The valve means includes a movable valve member having a first position permitting fluid communication between the first and second ports while isolating the third port. The movable valve member has at least one position permitting partial fluid communication between the first port and the second port and between the first port and the third port, the movement of the movable valve member being independent of the operation of the main control means.
In accordance with another aspect of the present invention, the movable valve member has a second position permitting fluid communication between the second and third ports while isolating the first port, and the position of the movable valve member is infinitely variable between the first and second positions whereby the pressure in the load signal chamber is infinitely variable between the load signal pressure and the reference fluid pressure, respectively.